1. Field of the Invention
This invention relates to solid state metal-oxide-semiconductor MOS integrated circuits and more particularly to an integrated resistor that has a very high resistance value and is thus particularly advantageous for use in a feedback path of an MOS amplifier.
2. Background Art
Resistors commonly used in complementary metal-oxide-semiconductor (CMOS) integrated circuits are constructed using polysilicon above the semiconductor surface or implanted layers below the surface of the silicon. Polysilicon resistors used in modem CMOS circuits are normally constructed as a pattern of squares by depositing a uniform polysilicon layer on the field oxide of the integrated circuit. This layer typically has a resistivity in the range of 20 to 40 Ohms per square in a modern 0.35 xcexcm process. Most of these processes include some way to block or exclude from the polysilicon resistor surface the silicide that would otherwise reduce the resistivity by an order of magnitude. Another common technique is to add a second polysilicon layer designated for use as a resistive layer with no silicide added. In both of these cases, polysilicon that is not intended to be a part of the resistor circuit is etched away using common lithographic techniques.
It is possible to construct a polysilicon resistor with a value as high as 10,000 Ohms, but then a moderately large amount of circuit area is required. Such a resistor would, for example, have to be constructed from 250 squares of polysilicon of 40 Ohms per square. Because of the large area required, such a resistor may also have a large capacitance to the substrate.
Implanted layers used as resistors are commonly constructed from the N or P implants that are used to construct source, drain or well regions. The well region implant has the highest resistivityxe2x80x94on the order of 1 kOhms per square. Such high-value implanted resistors, because of their intimate contact with the substrate, suffer from the limitations of both high parasitic capacitance to the substrate and high leakage current to the substrate.
Onboard analog CMOS circuits are commonly constructed with filter capacitance elements having values less than a pico-Farad (pF). Larger capacitors are possible, but require larger surface area. A capacitance value of 10 pF, for example, will require 10,000 square microns of circuit area for a typical specific capacitance of 1 femto-Farad (fF) per square micron.
For many years, CMOS analog circuitry has been based on switched-capacitor technology. One reason for this is that switched capacitor filters can be designed using small switched capacitor values in order to implement high-pass filters with band edges in the range of several kHz. To realize a continuous-time filter with a 1 kHz band edge f0 and a 1 pf capacitor C would require a resistor value of 1/(2xc2x7xcfx80xc2x7f0xc2x7C)=159 mega Ohms. The construction of such large resistor values using 150 squares of resistive material would thus require a resistivity of more than a million Ohms per square. In applications such as active high-pass continuous-time IC filters, the lack of very high resistor values restricts the filter range to very high cut-off frequencies. Moreover, any circuit based on switched-capacitor technology will, almost by definition, add noise and decrease the high-frequency performance of high-speed devices such as analog-to-digital converters and other operational amplifier-based circuits, and will often not be suitable useable in other continuous-time applications at all.
A resistive isolation layer with extremely high resistance is normally applied over the top layer of metal to seal an integrated circuit (IC) against moisture. This resistive isolation layer is typically formed from silicon nitride or silicon oxynitride. The insulation resistance of this layer is typically on the order of tens to hundreds of mega Ohms per square. This layer is, however, a uniform coating that is not shaped or patterned by any lithographic process. Consequently, any portion of the top metal layer contacting the insulating layer may be resistively connected to other top metal structures present on the integrated circuit (IC). Another problem with the resistive layer is that any charge-sensitive circuits formed by top metal connected to the resistive insulating layer may not be electrostatically shielded by covering with any additional metal layers.
One type of CMOS circuit that is particularly sensitive to stray capacitances and other stray charges are charge-sensitive amplifiers, which, by definition, will amplify any such unwanted, stray charges. This problem can be especially detrimental in circuits such as analog-to-digital converters (ADC), in which input and feedback charge control is essential in order to avoid an erroneous binary output.
What is needed is therefore a way to implement a resistive element that has a high resistance value but that does not require such a large circuit area to implement. The implementation should preferably electrostatically shield any charge-sensitive input terminal from the negative effects of various electrostatic fields that typically occur in such integrated circuits. Such a resistive element should also make it possible to implement a CMOS amplifier whose resistive feedback path is much less sensitive to stray charges than is now the case. This invention provides such a resistive element and amplifier implementation.
In its broadest terms, the invention is a high-value resistive element (xe2x80x9cresistorxe2x80x9d) that may be fabricated using the CMOS resistive isolation layer of a larger CMOS IC. This layer is also used to provide moisture isolation for the IC, especially circuits that are connected to the top metal that is also used in the resistor. The several elements of the resistor according to the invention provide both resistive and electrostatic shielding. This shielding is especially useful for shielding a charge-sensitive input terminal, for example, of an amplifier that includes the resistor. The shielding also reduces capacitive and resistive coupling from electrostatic fields that may arise not only between the resistor""s input and output electrodes and connections, but also from both external and internal stray current sources other than those at the output terminal. Such sources include substrate voltages and voltages connected to other metal layers, as well as other circuits fabricated on the same IC as the resistor.
In particular, the invention provides an integrated-circuit resistor that has an input, which includes an input electrode, and an output, which includes an output electrode. A substantially continuous, resistive sealing layer electrically connects the input and output. The resistive layer itself forms a resistive electrical path between the input and output; isolation of the resistor from other components on the same integrated circuit is preferably provided without patterning of the resistive layer. A top metal layer is in contact with the resistive layer; the input and output electrodes are formed as portions of the top metal layer. An output ring portion of the top metal layer is electrically connected to the output electrode and surrounds the input electrode.
In the preferred embodiment, a grounded shield ring portion of the top metal layer is preferably included that surrounds the output ring portion and forms a first conductive and electrostatic shield for the resistor.
A lower metal layer is preferably also included, portions of which form an input connection, which connects the input electrode with an external source, and an output connection, which connects the output electrode with an external circuit. A grounded intermediate metal layer is then preferably also included. This intermediate layer extends between the input connection and the output ring portion and forms a second electrostatic shield between the input connection and the output ring portion.
The invention also encompasses an integrated circuit (IC) amplifier that has the IC resistor in a feedback path.